What happens when a man finds “the one”? His brain gets washed in oxytocin.

The hormone and brain chemical best known for its role in love — it’s also responsible for helping infants bond to their mothers — can also make romantic partners look more attractive than strangers to men, even if both are objectively equally good looking.

Published in the Proceedings of the National Academy of Sciences, the study included 40 young men, all of whom had been in a relationship for at least six months and reported being passionately in love with their partners. While in a brain scanner, they either inhaled oxytocin or placebo via nasal spray while they viewed pictures of either their partners, women they knew but were not dating or women they had never met. The pictures were matched so that comparison women had been rated by independent observers as being equally attractive as the partners. In the men who were given oxytocin, the pleasure and desire regions of their brains lit up when they saw pictures of the women they loved — but not when they looked at strangers. Some of these regions were also activated by the images of the women the men knew, but not as strongly as by the pictures of their loved ones, suggesting that it made their partners more desirable.

“It’s really intriguing,” says Larry Young, professor of psychiatry at Emory University, in Atlanta, who was not associated with the research. The study is one of the first to show a role for the hormone in human monogamy (prior research revealed similar effects in other mammals).

How monogamy works has long been a biological mystery. “Sexual monogamy in humans is potentially costly for males,” says lead author Dr. Rene Hurlemann, professor of psychiatry at the University of Bonn in Germany, explaining that he and his team wanted to understand some of the chemical contributors to the practice. Only 3% of mammals are monogamous, and that small proportion likely reflects the fact that from a simple biological perspective, it makes little sense for males who could produce far more offspring by mating with multiple females.

“Once men receive oxytocin, the attractiveness of the partner increases compared to the attractiveness value recorded for other females,” says Hurlemann. And the men were not aware that they behaved differently on oxytocin; in fact, they could not reliably distinguish between it and placebo. They didn’t feel “high” or any “craving” — at least as measured by their ratings of their feelings in both conditions.

“It really is very subtle,” says Young, noting that the size of the difference between oxytocin and placebo was small. Whether this is because intranasal oxytocin doesn’t have a powerful effect on the brain or because oxytocin typically changes behavior without our conscious awareness is not known.

What is clear, however, is that oxytocin can create unconscious biases in favor of a partner, possibly providing part of the biological mechanism behind monogamy. A prior study by the same researchers, in fact, found that men in monogamous relationships who were given oxytocin actually kept a greater physical distance from an attractive research associate, compared with single men.

Young suggests that oxytocin may actually have a dual effect — by not only making partners more attractive but also actively deterring interest in other potential mates. He notes that in the monogamous prairie voles he studies, males that have a pair bond can actually be hostile to other females. “They develop a very strong preference for the partner and slight aggression towards those who are not their partners,” he says.

The fact that these biases are only seen in men when under the influence of oxytocin — not placebo — may even hold lessons for those who want to stay faithful. “Think about when oxytocin is released,” says Hurlemann, noting that this occurs during kissing, hugging, orgasm and other intimate moments. The more such moments you have, the more oxytocin is released in the body. And, says Young, if you combine this with other rewarding experiences that also get the reward system going — such as doing new and challenging things together — you can strengthen your connection to each other. Seems like oxytocin might be the perfect ingredient for a fragrance called Faithful.